Employment of high voltage direct current (HVDC) transmission for transmitting a large amount of electric power over a long distance is progressing. For example, in an offshore wind turbine, AC power transmission is generally performed when an offshore distance is short (equal to or shorter than several km), whereas DC power transmission is used when the offshore distance is long.
In a case in which DC power transmission from electric power generation equipment such as an offshore wind turbine to an electric power system is performed, when an incident such as a short-circuit, grounding or the like is generated at a point of interconnection with the electric power system and thus the voltage of the interconnection point decreases, it may not be possible to transmit electric power to the electric power system. Meanwhile, when no countermeasures are taken in the electric power generation equipment such as a wind turbine, the electric power generation equipment continues electric power generation because the electric power generation equipment cannot detect the incident. As a result, the electric power of a DC power transmission unit becomes excessive and the voltage of the DC power transmission unit increases. Due to such voltage increase, faults such as device breakage may be generated.
To prevent such voltage increase, a method for installing a damping resistor and a semiconductor switch in the DC power transmission unit to dissipate excessive electric power may be conceived. However, to dissipate all excessive electric power, the capacities of the damping resistor and the semiconductor switch also become significantly high resulting in increases in the costs, weights and sizes thereof. This causes an increase in the electric power generation costs of the whole electric power generation system including the DC power transmission unit.
In view of this, Patent Literature 1 discloses a method for decreasing an AC voltage of an offshore converter in order to limit electric power from an offshore wind turbine which flows into an offshore AC/DC converter as a method for decreasing the capacities of a damping resistor and a semiconductor switch for the offshore wind turbine.